The goal of this project is to develop and test fully 3-dimensional electron beam dose calculation system which uses Monte Carlo techniques. Monte Carlo transport techniques represent the most accurate physical model available but previously have been considered too time consuming for routine treatment planning on hardware typically available in radiotherapy centers. With major breakthroughs occuring in computing hardware, this will no longer be a constraint. Monte Carlo techniques are needed in electron treatment planning. Even the most advanced dose-computation models are known to make errors of greater than 10% near inhomogeneities. Much of the software developed will be applicable to dose computation in photon beams, although for photon beams the errors in analytic models are less severe and the time for Monte Carlo simulation is much longer. There are four specific components of this project: 1) The development of highly efficient and accurate Monte Carlo codes for calculating 3-dimension dose distributions in individual patients, starting for CT data and the description of the clinical beam and any beam modifying devices; 2) The development of techniques for characterizing clinical electron beams for input to the treatment planning routines; 3) The demonstration within a 3-dimensional radiotherapy treatment planning system for routine Monte Carlo computation of clinical electron- beam dose distributions using the new generation of very fast but inexpensive computers; 4) The validation of the technique using a variety of experimental phantoms and clinical examples.